Abstract The phase diagram of the LaCu system was reinvestigated by DTA in the concentration range 0–33 at.% La. The crystal structures of the various intermetallic compounds were examined by X-ray diffraction on powdered or single-crystal specimens. Two new phases, X (7.5 at.% La) and LaCu 4, were observed besides the well-known compounds LaCu 6, LaCu 5 and LaCu 2. X forms by peritectic reaction (873 °C) and decomposes on cooling. LaCu 4 forms by peritectic reaction (743 °C). Its crystal structure is tetragonal (space group I 4 ̄ m2 ) with a = 0.8904 nm, c = 1.9210 nm and Z = 18.